The present invention relates to a novel resin composition based on a polypropylene. More particularly, the invention relates to a novel resin composition based on a polypropylene which is excellent in respect of the adaptability to blow molding such as draw-down resistance, pinch-off characteristic, deep-drawability and the like and capable of giving molded articles having excellent mechanical properties to satisfy the requirements, for example, for bumper beams of automobiles and has an additional advantage that the resin composition is suitable for reclaiming wastes of once molded articles to ensure outstandingly low costs.
As is known, automobile bumper beams are traditionally shaped of a metallic material but, ;n recent years, metal-made bumpers are being rapidly replaced with plastic resin-made ones to comply with the requirements for lighter weight, higher rust-proofness and better moldability. Various resin compositions have been proposed and are under practical use in the prior art as a material of automobile bumper beams including those based on, for example, a high-density polyethylene, polypropylene, Xenoy resin which is a polymer blend of poly(butylene terephthalate) and polycarbonate manufactured and supplied by General Electric Co., talc-containing polypropylene-based composite material, polypropylene-impregnated glass fiber mat in the form of a stampable sheet, SMC resin and so on.
Each of these resins or resin compositions in the prior art has its own advantages and disadvantages in view of the requirements in recent years for upgraded properties of automobile bumper beams to meet the very strict standard in the United States for automobile bumper beams including the so-called 5-miles/hour pendulum test if not to mention the costs. For example, bumper beams shaped by molding a high-density polyethylene or polypropylene are defective due to the poor mechanical strengths although they are advantageous in respect of the low costs. The automobile bumpers prepared by blow-molding of the Xenoy resin are somewhat too expensive due to the material cost and not suitable for reclaiming wastes of once molded bodies. The talc-containing polypropylene-based composite material, which is suitable for blow molding, can only give bumper beams having poor impact strength at low temperatures. The total cost of bumper beams is unavoidably high when they are shaped from the polypropylene-impregnated stampable sheet or from the SMC resin.